This invention relates to a supply valve for use in a dual circuit braking system.
Double check valves for dual circuit systems operate to communicate either of two fluid pressure supplies to a circuit associated with a vehicle air brake system. Double check valves are used between the two service reservoirs in many air brake systems to supply air pressure to the parking and trailer brake circuits. Tests on typical braking systems have shown that leakage rates of up to 50 psi per minute are required to cause a standard double check valve to isolate a leaking reservoir. This is because at lower leakage rates, the pressure differential between the inlet ports of the check valve is insufficient to cause the shuttle used in the check valve, which responds to a pressure differential between the inlet ports, to seal the inlet port which communicates to the leaking reservoir. As a result, the shuttle "stalls" in the mid-position allowing air to "flow by" from an intact reservoir to a leaking reservoir.
Vehicle tests have shown that even with leakage rates approaching 50 psi, emergency stopping distance requirements can be met. Clearly, however, a valve that does not permit "flow by" would represent an improvement, since it would insure that maximum pressure is maintained in one of the supply circuits after a failure in the other supply circuit.